4x4ord wrote:
Hannibal wrote:
shepstone wrote:
I don't quite understand why max hp and max tq are both achieved at different rpm ? and with that which of the 2 rpm's should I be using to get the most out of an engine when climbing or accelerating with a heavy load?
Max horsepower will put the most torque to the rear wheels.
It kind of depends how you look at this question. Say you start climbing a hill with a big trailer. As the grade increases you floor it and your transmission drops two gears and the engine is running at near the rpm where it makes maximum power. The hill is long and getting steeper, you keep your foot to the floor but the engine continues to loose rpm; as it slows it continues to increase the torque to the rear wheels until it either meets the demand of the hill or the rpm drops to where the e.engine is making maximum torque. If the hill still demands more torque another downsjit is required and the engine increases its rpm again so it can start the cycle over again. Anyway you will pull the hill the fastest in the gear that keeps your engine revved close to the rpm where maximum HP is achieved but in each gear the rpm where the engine makes the most torque is always the rpm where the engine will pull the hardest in that gear.
Very well written!
To add a bit more: When you start to slow the key is whether the engine continues to produce the same (or more) torque as the RPM drops. If the torque drops off as RPM does you will continue to slow, possibly to the point of downshifting. This is why a flat or inverse torque curve is what really makes a high-torque engine perform well. 1000 lb-ft of peak torque wouldn't be very useful if it disappeared when you slowed the engine a bit.
Now if the transmission provides plenty of closely-spaced gears the shape of the torque curve becomes less important. Rather than having to provide the same torque at lower RPM the transmission can just select the next lower gear and re-increase the RPM.
This is why with the limited gears choices available when climbing a grade, equal HP does not always translate to equal performance.
